AMSTERDAM (AP) - Thieves broke into a Rotterdam museum on Tuesday and walked off with works from the likes of Picasso, Monet, Gauguin and Matisse potentially worth hundreds of millions.

Police haven't said how they pulled off the early hours heist, but an expert who tracks stolen art said the robbers clearly knew what they were after.

"Those thieves got one hell of a haul," said Chris Marinello, who directs the Art Loss Register.

The heist at the Kunsthal museum is one of the largest in years in the Netherlands, and is a stunning blow for the private Triton Foundation collection, which was being exhibited publicly as a group for the first time.

"It's every museum director's worst nightmare," said Kunsthal director Emily Ansenk, who had been in Istanbul on business but returned immediately.

News of the theft "struck like a bomb," she said at a press conference in the museum's cafe.

She declined to reveal any details of how the thieves struck, or how the museum is protected, other than describing its security as "state of the art" and "functional."

Willem van Hassel, the museum's chairman, said its security systems are automated, and do not use guards on site.

Police arrived at the scene five minutes after an alarm was triggered, he said. He described the museum's insurance as adequate for the exhibition.

The collection was on display as part of celebrations surrounding the museum's 20th anniversary.

Police spokeswoman Willemieke Romijn said investigators were reviewing videotapes of the theft, which took place around 3 a.m. local time, and calling for any witnesses to come forward.

The Art Loss Register's Marinello said the items taken could be worth "hundreds of millions of euros" if sold legally at auction. However, he said that was now impossible, as the paintings have already been registered internationally as stolen.

Marinello said the thieves have limited options available, such as seeking a ransom from the owners, the museum or the insurers. They could conceivably sell the paintings in the criminal market too, though any sale would likely be a small fraction of their potential auction value.

Dutch forensic experts took a sample of imprints at Kunsthal Museum. The museum is closed today because of the theft.
Asenk said she spoke on behalf of the family in saying "we are shocked, but we will go on."

"All involved want the public to still be able to see these kinds of special collections and private collections," she said.

The museum was cordoned off as police carried out their investigation Tuesday, but the museum will reopen Wednesday, she said.

The Kunsthal museum is a display space that has no permanent collection of its own - the name means "art gallery" in Dutch.